It’s the year 2009. After two decades of doing mostly object-oriented programming the claim that functional languages will greatly simplify the writing of correct concurrent programs gets me interested in Clojure, Erlang and eventually Haskell. Today, in 2018, my activities in concurrency have somewhat faded away, but my curiosity about the functional side of software development has stayed with me.

As a developer I am being test-driven through and through, so a community’s approach to testing is one of the first things I look at when trying to grasp how “the others” work and tick. That’s how I learned that functional programming folks are very much into properties as opposed to what we OO programmers call unit tests. This change of perspective cannot only be seen in strictly typed languages like Haskell but also in dynamically typed ones like Erlang.

However, Java, Groovy and to a lesser degree JavaScript are still my working horses when it comes to earning money. Thus I began to wonder whether Property-based Testing (PBT) can help me here, too. Googling DuckDuckGoing for “Java” and “Property-based Testing” will give you a few results - some dating back to the time when JUnit’s Theories-Runner was still a thing - but there is much less to be found on the web than I had expected. Some people e.g. Nat Price were experimenting with the approach in Java, but that was about it.

Call it a lucky coincidence that at this time - two years ago - I left the JUnit-5 core team but decided to experiment with the JUnit platform. That’s why I created a test engine called jqwik with a strong focus on property-based tests. Besides diving into the intricacies of Java’s completely messed-up approach to reflection, this task also required that I experimented with different ways to write properties, that I looked at how other PBT libraries worked, and that I found a way to integrate these new type of tests into my well-dried style of TDD.

In the weeks to come I’ll publish a series of blog entries describing the essence of what I learned about PBT on the way, and how one can use it in Java. So far, eight parts have been published:

I hope to see you again on my journey!